The major reason companies are staying private longer these days is that it takes them longer to be ready to be public.
In order to be ready to be public company you have to match the risk-return preference characteristics of public investors, i.e. you need to be understandable for that investors’ segment, and the most important factor for that is to have a clear path to stable profitable growth. This takes longer to achieve during the age of many new “winner takes most” business models which many tech companies are pursuing. When Microsoft went public it was more ready to be a public company at valuation of 580 million than Uber was at the valuation of over $50bn, because Microsoft had a clear path to profitable growth, it actually was already there. While Uber, Snapchat and many others still have lots of unknows about their future financial performance which makes lesser match for public market investors.
Some of the other factors listed in the article are playing role too, like more availability of private capital which surged due to low interest rate environment, but the driving factor is still less readiness due to uncertain business models which make future financial performance more difficult to evaluate